The present invention relates to a communication system having radio access units connected to optical fibers.
Conventionally, a wireless local area network (LAN) is used indoors for radio communications between computer terminals. The wireless LAN involves no wire connection of a terminal to a LAN connecting port, and hence it provides greater flexibility in the placement of terminals than does LAN that requires wire connection between computer terminals.
The wireless LANs known so far are, for example, a radio system in the unlicensed ISM (Industrial Scientific and Medical) band at 2.4 GHz using a spread spectrum scheme, a radio channel access method using OFDM (Orthogonal Frequency Division Multiplexing) scheme at 5 GHz according to IEEE802.11 and IEEE1394, and the Buletooth (short distance radio communication scheme) using the spread spectrum scheme based on the frequency hopping system.
These wireless LANs mostly employ such a star network as shown in FIG. 1. The star network has a center node 300 at the center of the network and plural nodes 310 to 340 connected to the center node 300. There is also used a combinatorial wireless LAN wherein multiple center nodes of such star networks are connected by cables.
On the other hand, there has recently been put to practical use an indoor transmission system that permits the use of portable telephones and mobile stations in dead zones such as underground shopping areas, buildings and tunnels (Japanese Pat. Laid-Open Gazette No. 284837/97). The indoor transmission system comprises, as depicted in FIG. 2, a base station unit 200, radio access units 210a to 210n, and optical fibers 220a and 220b. 
The base station unit 200 comprises a mobile radio modem 201, an E/O (Electrical/Optical) converter 202 for converting an electric signal to an optical signal, and an O/E (Optical/Electrical) converter 203 for converting an optical signal to an electric signal. The base station unit 200 and the radio access units 210a to 210n are connected to the optical fibers 220a and 220b. The radio access units 210a to 210n have O/E converters 211a to 211n for converting an optical signal to an electric signal and E/O converters 212a to 212n for converting an electric signal to an optical signal.
In FIG. 2, a radio-frequency signal (an RF signal) sent from a mobile station 300 is received, for example, by the radio access unit 210a, wherein it is converted by the E/O converter 212a to an optical signal. The optical signal is sent via the optical fiber 220b to the base station unit 200, wherein it is converted by the O/E converter 203. The signal thus converted to an electric signal is demodulated by the mobile radio modem 201 as predetermined for connection to a mobile communication network 70.
On the other hand, a signal from the mobile communication network 70 is modulated by the modem 201 as predetermined and converted by the E/O converter 202 into an optical signal, which is sent via the optical fiber 220a to the radio access units 210a to 210n. The radio access units 210a to 210n convert the received optical signal by 211a to 211n to an electric signal, and radiate radio waves to mobile stations 300. The mobile stations 300 receive the RF signals.
In the conventional system of FIG. 2, since radio access units send the same down-link radio signal, the radio zone configuration is virtually a single cell. On this account, the subscriber capacity of the indoor radio system is limited as compared with an outdoor radio system.